No-load test: Apply the rated voltage of the sine wave rated frequency from a certain winding of the transformer (generally from the secondary low-voltage side), and open the other windings to measure the no-load current and no-load loss. If the test conditions are limited and the power supply voltage cannot reach the rated voltage, the test can be performed under non-rated voltage conditions. This test method has a large error and is generally only used to check whether the transformer is faulty. Only the test voltage reaches more than 80% of the rated voltage. can be used to test the no-load loss.